Photon ring multi-user distribution system for soft X-ray SASE FEL laboratory

It is shown that although the soft X-ray SASE FEL photon beam from undulator is in principle a single user tool, just like an optical laser, the optical distribution system based on multifacet reflectors provide efficient ways to generate a multi-user facility--very similar to present-day synchrotron radiation facilities. Multifacet reflectors involve multiple reflections from a series of plane mirrors. These make use of the principle of total external reflectance. The candidate mirror materials include C, Mo, and Si which should yield a reflectance of 40-80% for an arc angle of 90 deg. in the wavelength range 6-100 nm. Multifacet reflector arcs can be repeated a number of times to form a complete ring. In order to make efficient use of the new SASE FEL source, it is proposed to install this photon ring distribution system in the experimental hall of soft X-ray FEL laboratory. A few tens beam lines with different experiments can be served by a single FEL source. Using movable mirrors in each photon ring cell, it is possible to quickly switch the FEL photon beam from one experiment to the other, thus providing simultaneous multi-user capability